A hundred days is a journalistic conceit. The closest parallel is Franklin Roosevelt, who used his first days to bombard Congress with measures to confront the Great Depression, but Roosevelt only had to deal with a domestic crisis, not two wars as well. It is too soon to judge Barack Obama's presidency. However, it is not premature to assess either the scale of his ambition or his vision. Nor is it right to lapse into the comfort zone of cynicism. Things are bound to go wrong. Some of the many balls he has thrown in the air will come crashing down. But that does not invalidate the purpose.

First, the scale of his ambition: closing Guantánamo Bay and the secret prisons around the world; confronting the CIA with the release of the torture memos; turning against the concept of America as the national security state; the $789bn stimulus bill; major interventions in housing and credit markets; offering Iran a new relationship; pressing the reset button on Russia; defining an exit strategy on Iraq; redefining the war in Afghanistan; appointing a world-class negotiator on Israel-Palestine; climate change, Cuba, and a major speech on Islam in Turkey ... the list is long. And its very length has exposed two myths - that Mr Obama was doomed to continue the policies of George Bush, and that he would only be able to tackle one crisis at a time. Neither has proved true.

Each item on the Obama agenda constitutes a shift in its own right, in some cases reversing not just Bush-era policies but Clinton-era ones as well. The cumulative effect is to overhaul US foreign policy, no less, while trying to tackle the greatest financial crisis since 1929. The president's ambition has not gone unnoticed in Beijing, Moscow or Tehran, nor in capitals less wary of US leadership. America is making major strides to regain its rightful place on the world stage, but one acquired by respect and negotiation rather than by force alone. And that matters for European states which cannot get their act together.

Now come the qualifications. If there are three phases to any diplomatic conflict - rhetoric, negotiation and settlement - Mr Obama is still only at the stage of rhetorical intention. It remains to be seen how much he will give at the climate change summit in Copenhagen. There are also large questions about what he will get in return. An outstretched hand requires a response, and Iran's will not be swiftly given. Russia has different policy objectives from America, and will never return to the days when Boris (Yeltsin) curried favoured with Bill (Clinton) by handing over secrets of state, such as the latest tank. Then there are the initiatives themselves. Despite the fact that the collapse of Lehman Brothers contributed to his victory, Mr Obama has been at his weakest tackling the banking crisis. A $789bn stimulus sounds large, but may not be enough to cope with the scale of the crisis in the US. The package has some wasteful measures and stupid giveaways such as government money to buy houses, not just for first-time buyers, but any houses. Mr Obama's lieutenants Tim Geithner and Larry Summers have been too timid with the masters of the universe, handing failed institutions taxpayer dollars instead of nationalising them. The latest idea is to give financiers public money as an inducement to trade toxic assets. That is right: a crisis caused by speculators is now apparently to be solved by speculators. No wonder American voters are angry.

For all that, Mr Obama stands almost as high in public opinion as he did a hundred days ago. He has opponents but no opposition. The Republicans are leaderless and likely to remain so for some time to come. Karl Rove, Dick Cheney and Newt Gingrich really are characters from the past. The Obama brand has done what it says on the tin and produced a leader of candour and vision. The real challenge of government still lies ahead, but for the moment he is simply the coolest politician on the planet.